1. Field of the Invention
The present invention relates to an optical rotary encoder used to control a speed of a rotary machine or a rotation position, and more particularly to a rotary encoder having a photosensor for compensating a variation in an output of a light emitting device.
2. Description of the Prior Art
FIGS. 1, 2 and 3 show one example of a conventional optical rotary encoder. Numeral 1 denotes a rotating slit disc mounted on a rotating shaft 2. As shown in FIG. 2, the disc 1 has one slit 1A and a number of slits 1B formed along a periphery thereof. A stationary slit plate 3 arranged to oppose to the periphery of the disc 1 has stationary slits 3A, 3B and 3C formed at overlapping positions to the slits 1A and 1B. (See FIG. 3). As is well known, the slit 1A is used to detect a home position in each revolution and the slits 1B are used to detect, for example, type positions.
Numeral 4 denotes a light emitting device and numeral 5 denotes a plurality of photosensors. As the disc 1 is rotated, each time the slits 1A or 1B overlap the stationary slits 3A, 3B and 3C, the light from the light emitting device 4 passes through the slits 1A or 1B and the slits 3A, 3B and 3C and impinges to the photosensors 5. A change in the intensity of light sensed is converted to an electrical signal so that a control is made in accordance with the output signal.
As shown in FIGS. 2 and 3, the photosensors 5 include a monitor photosensor 5M whichis arranged at a position where the light from the light emitting device 4 is received without routing the slits 1A and 1B and the slits 3A, 3B and 3C. The light output from the light emitting device 4 is monitored by the monitor photosensor 5M.
Since the output of the light emitting device 4 changes with an environmental temperature or aging, the monitor photosensor 5M is provided to detect the light intensity in order to compensate such a change. Thus, the light output from the light emitting device 4 is controlled by a drive circuit (not shown) for the light emitting device 4 such that the light intensity at the monitor photosensor 5M is always kept constant.
In such a prior art rotary encoder, however, since the light from the light emitting device 4 is reflected by the rotating disc 1 or scattered or disturbed before it is sensed by the photosensor 5M, the sensed light intensity may change in spite of no change in the light output.
In the prior art rotary encoder, the slit 1A is formed in the rotary slit disc 1 by vapordepositing a metal film on one side of a glass plate except the slit area. A certain degree of decentering at a periphery of the disc 1 is usually admitted because high precision of coaxiality between the slit pattern and the periphery of the disc 1 would lead to the increase of cost.
Because of the decentering of the periphery, the light from the light emitting device 4 may be affected during the rotation of the disc 1. As a result, the light intensity sensed by the monitor photosensor 5M may change in spite of no change in the light output.
Thus, the light output apparently changes and the intended purpose of compensating the change in the light output from the light emitting device 4 is damaged and the light output from the light emitting device 4 is unnecessarily changed and a stable control is not attained.
In order to resolve the above problem, it may be possible to arrange the monitor photosensor which senses the light intensity of the light emitting device 4 on the side of the slit disc 1 on which the light emitting device 4 is mounted in such a manner that the light from the light emitting device 4 is directly sensed by the monitor photosensor in order to prevent the light emitted from the light emitting device 4 and sensed by the monitor photosensor from being affected by the rotation of the slit disc 1.
However, the above arrangement does not resolve a problem of affect by external optical noises and variations in the mounting position and direction of the monitor photosensor with respect to the light emitting device 4 affect the light intensity sensed by the monitor photosensor so that an adjustment for compensating the change in the light output is complicated and work amount is increased.
One of factors which significantly affects the characteristic of the output signal is a distance or gap between the rotating slit disc 1 and the stationary index slit 3. The larger the number of slits is, the smaller must be the gap and the more is the affect to the output signal by the change of the gap. Accordingly, in the encoder having a large number of slits, it is necessary to reduce the gap and keep the gap constant.
On the other hand, since the encoder is usually directly coupled to a rotary machine, it is directly subjected to a heat generated by the rotary machine.
An encoder case which has a complex shape is, in many cases, molded in order to prevent cost increase. In this case, because of a difference between coefficients of linear expansion of the rotary shaft 2 of the rotary machine and the encoder case, dimensions in motor thrust direction at a low temperature and a high temperature are different. Thus, the gap substantially changes and the output signal largely varies with the temperature.
In order to reduce the above phenomenon, the thrust dimension of the encoder case may be reduced to lessen the affect by the coefficient of linear expansion. However, the thrust dimension cannot actually be reduced because of the limitation by the size of the light emitting device 4 and the fact that a larger light emitting device offers a more stable optical characteristic. Accordingly, the size of the encoder case cannot also be reduced.
In the past, the monitor photosensor used to maintain the light output from the light source of the optical encoder at a constant level is constructed as a separate part from the light emitting device. Accordingly, the distance between the light emitting device and the photosensor is long, the light intensity sensed by the photosensor is insufficient or the photosensor may sensor an external light accordingly, it is necessary to optically sealed those parts.
Because the light emitting device and the photosensor are separate parts, it is necessary to adjust a distance between those parts and orientation of those parts when they are assembled.